Illuminating Our World:
A study of the Quantum Dot LED
EE453 Project Report submitted by
Mark Adams, , Fall 2008
In the movie “Indiana Jones and the Kingdom of the Crystal Skull,” Indiana’s Sidekick Mutt is stung by a scorpion. When told that it was a very big scorpion, actor Harrison Ford said, “When it comes to scorpions, the bigger the better. Small one bites ya, don't keep it to yourself[i].” This is similar to the way that quantum physics works. The bigger an object or particle, the less and less you see quantum effects, but the smaller and smaller, the more and more you see quantum effects.
Nanotechnology is a field of science and engineering that deals with particles smaller than 100 nm. When a particle is smaller than 100 nm, you begin to see a special effect. For starters, the reactive part of any matter is its surface area. The larger a particle gets, the smaller the surface area gets relative to its volume. The smaller a particle gets, the larger the surface area gets relative to its volume. There is something called the Photoelectric effect, in which quantum particles stand out from the rest. Wikipedia describes the photoelectric effect as “a quantum electronic phenomenon in which electrons are emitted from matter after the absorption of energy from electromagnetic radiation such as x-rays or visible light. The emitted electrons can be referred to as photoelectrons in this context[ii].” When a particle is exposed to light, as long as the light above a certain threshold frequency, electrons will be emitted as shown below:
The part where nanoparticles stand out is that photons will not be released at a certain threshold, they will be released only at certain wavelengths.
The basic idea being that the smallest size of a photon is a single electron. The next smallest is two electrons, then three, and so on. You can’t have a photon of 1 ½ electrons, or 1 1/3 electrons, or 1 ¼ electrons. Large particles have a wide valence band and a wide conduction band. Nanoparticles have a narrow valence band and a narrow conduction band. For a larger particle, the wavelength of light can be a large range and still have that distance be from somewhere in the valence band to somewhere in the conduction band. While a particle larger than nanoscale will emit photons any time that the incident wave is above the threshold wavelength, the nanoparticle will only emit energy while the incident wave is exactly the band gap from one valence band to one conduction band, as the example below illustrates
Quantum dots are a special type of nanoparticle. They are sphere shaped, as opposed to square, rectangle, wire-like, etc. They are usually composed of two materials or two layers. The inner layer is a reactive material like gold, and the outer layer is a covering to prevent the quantum dots from joining together to form larger particles.
An important property of quantum dots is the fact that the energy levels depend on the size of the quantum dot itself. One important application is the advancement of LED Technology.
L.E.D. is the abbreviation of Light Emitting Diode. A diode is a PN junction. It consists of a region of semiconductor like silicon doped positive and one that is doped negative. When a voltage is applied forward bias above a certain threshold, current will flow.
An LED is a special kind of diode. When it is forward biased, and a current is flowing, photons, i.e. light, are released as part of recombination[iii]. This process is called Electroluminescence.
There are several types of light on the market today. Perhaps the most well known is incandescent lighting, first made practical by Thomas Edison. It uses a filament, or a resistor, housed in a special gas enclosed in a bulb. This type of lighting is widely used but its main drawback is that it uses a lot of electricity, and produces a lot of heat, and the filament has a relatively short life compared to other options. Another type is fluorescent or neon, it uses much less electricity per amount of light, and is cooler. Finally there are LED’s of which this paper has mentioned. They are substantially more power efficient, and in some cases, more efficient than fluorescent. When properly used and quality constructed, they last for much, much longer than any other option.
LED’s are being used and researched for several applications, they are good for some applications but not good enough for others. LED’s are being researched to being used to replace fluorescent lighting in offices and homes. One difficulty here is that in one aspect of the design they seem to have room for improvement. A popular method of producing white light is by coating the LED with a phosphor to change the spectrum out output light. According to Wikipedia, “Phosphor based LEDs have a lower efficiency than normal LEDs due to the heat loss from the Stokes shift and also other phosphor-related degradation issues. However, the phosphor method is still the most popular technique for manufacturing high intensity white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex RGB system, and the majority of high intensity white LEDs presently on the market are manufactured using phosphor light conversion[iv].”
The answer to this question is through the use of quantum dots in LED’s. As has been stated, the energy levels in a quantum dot depend on the size of the quantum dot itself. What that means is that the spectrum of the output light, only depends on the size of the quantum dot. It’s like a different approach to an old problem.
How quantum dot LED’s work is they contain a light-emitting chip that emits light at the 400 nm range. Surrounding this is a dome where all the 400nm light will pass through. At the surface of the dome, covered in epoxy, are the quantum dots[v]. The size of the quantum dots is carefully chosen to be able to convert the 400 nm wave to the desired wavelength.
There are may possible applications for quantum dot LED’s. Obviously they can become a new standard in energy efficient lighting to illuminate homes and offices nationwide. As easy as it is to make them different colors without compromising efficiency, they may be a good candidate for the next generation of displays[vi]. The industry providing lighting for planted aquariums, aquarium coral reef aquariums, and hydroponics relies on being able to supply sufficient intensity in diverse spectrums of light. Current solutions involve high-light output, which gets the job done, but is not very efficient. The way quantum dot technology allows for high flexibility in spectrum without sacrificing efficiency, it may could take over the industry.
Although quantum dots LED’s exist and are on the market, there is a key improvement that will make them much better. The way quantum dots are used, as seen in the figure above, is not to provide the luminescence, but merely to shift the wavelength of the light from one wavelength to another. The better thing would be to actually use quantum dots to emit light. MIT researchers have been researching quantum dot technology for specific applications including that of replacing LCDs with quantum dots. According to an article entitled Quantum-dot LED may be screen of choice for future electronics[vii], “Unlike traditional LCDs, which must be lit from behind, quantum dots generate their own light. Depending on their size, the dots can be "tuned" to emit any color in the rainbow. And the colors of light they produce are much more saturated than that of other sources.”
The promise is that Quantum Dot LED’s are the most efficient LED’s is not exactly true, at least not at the moment. They appear to have potential for vast improvement over other technologies, below is a chart for comparison.
Light / Output/Input (Lumens/Watt)60-100 Watt Incandescent Light Bulb / 15 [viii]
Standard Fluorescent Lighting / 100 [ix]
Cree, Inc. White Light introduced in 2003 / 65 [x]
Cree, Inc. White Light 2006 prototype / 131
QD Vision QD-LED, 2006 / 3.4[xi]
QD Vision Green QD-LED, 2006 / 2.7[xii]
EvidotLED’s / Information not available
Currently Evidot is marketing their quantum dot LED’s and even LED products featuring their Quantum Dot LED’s, including Christmas lights, they aren’t boasting the actual luminous efficiency of their LED’s. It must be supposed that it is not significantly better than those in the table above. Currently quantum dots has great potential, but has not yet reached the pinnacle of efficiency.
Works Cited
[i] http://www.imdb.com/title/tt0367882/quotes
[ii] http://en.wikipedia.org/wiki/Photoelectric_effect
[iii] http://en.wikipedia.org/wiki/Electroluminescence
[iv] http://en.wikipedia.org/wiki/LED#Efficiency_and_operational_parameters
[v] http://www.physlink.com/News/071403QuantumDotLED.cfm
[vi] http://web.mit.edu/newsoffice/2002/dot.html
[vii] http://web.mit.edu/newsoffice/2002/dot.html
[viii] http://en.wikipedia.org/wiki/LED#Efficiency_and_operational_parameters
[ix] http://en.wikipedia.org/wiki/LED#Efficiency_and_operational_parameters
[x] http://en.wikipedia.org/wiki/LED#Efficiency_and_operational_parameters
[xi] http://www.businesswire.com/portal/site/google/?ndmViewId=news_view&newsId=20061113006263&newsLang=en
[xii] http://www.hcp.com/news/newsdetails.php/id/8734